Such devices have been known for many decades (German Patent Specification No. 179 675, German Patent Specification No. 224 700, German Patent Specification No. 873 908). They are all based on the following principle.
In a shallow housing having a viewing window which may, if desired, be covered by a transparent plate, there is a pile of pictures, the uppermost one of which is exposed to view through the viewing window and is supported at its edges. A pressure arrangement using a spring holds the pile pressed up against the window or the frame thereof. Laterally, the housing has an aperture through which a slider member may be pulled; the withdrawal movement of the slider member is limited by stops. Near to the housing aperture there is a member referred to here and hereinafter as a separator which extends the width of the aperture and which at the top and the bottom leaves clear a narrow slot for a picture to pass through. When the slider member is pulled out, the uppermost picture is pulled away through the upper slot by a hook-like transporter, the remainder of the pile being held back by the separator, and during the return movement of the slider member the picture is pushed through the bottom slot under the pile again or, more accurately speaking, between the pressure arrangement and the bottom-most picture of the pile. The second uppermost picture is now in front of the viewing window and the process can be repeated as often as desired.
This principle is extremely useful for perfectly flat thick plates, as mentioned in the first-named publications, because the transporter hook then engages sufficiently deeply on the edge of the plate and does not slip off. But with paper pictures, for instance photographic prints, having a thickness, for example of 1/4 mm or even less, there is a high probability that the hook, which itself must have a depth of penetration that is less than the smallest picture thickness, will slip off the edge of the picture or will not grip it at all, especially when the pictures have edges trimmed at an angle or if they are curved or distorted. The hook slipping off may lead to the hook scratching over the viewing side of the picture and damaging it. Furthermore, even when the carrier material of the picture is pliant, the hook will in the course of time wear down its clear-cut edge and will then be even less able to grip; in addition, the pliant picture carriers become deformed, twisted and possibly even torn at the side at which the hook engages, with the result that the device then no longer functions at all with these pictures.
It is an advantage in this design that the overall depth in the direction of the pile thickness is relatively small.
With a stereo viewer according to German Patent Specification No. 864,759, a picture is removed from a pile by means of a roller turned by hand and is conveyed beneath the pile again by means of a second roller. Admittedly, damage to the picture is here avoided, but the overall depth of the viewer is inevitably very much greater than in the case of the slider member design.